1. Field of the Invention
The present invention relates to a limiter for controlling an overvoltage and a radio frequency identification (RFID) tag having the same. More particularly, the present invention relates to a limiter for controlling an overvoltage to protect driving elements of the RFID tag by limiting an overvoltage received from an RFID reader.
2. Description of the Related Art
An RFID system is an automatic identification and data capture (ADC) technology using a radio frequency. The RFID system consists of an RFID reader and an RFID tag communicating with each other.
Specifically, the RFID reader transmits an electromagnetic wave of a certain frequency band and the RFID tag stores authentication information for identifying RFID tags and certain data. If the RFID tag is located within a magnetic field or electrical field of the RFID reader, the RFID tag transmits the authentication information and data to the RFID reader. The RFID reader authenticates the RFID tag using the authentication information.
The RFID tag receives power driving the RFID tag through an electromagnetic wave from the RFID reader. The intensity of the power from the RFID reader to the RFID tag corresponds to the distance between the RFID reader and the RFID tag. That is, the shorter the distance between the RFID reader and the RFID tag is, the more intense the input power from the RFID reader is, so that an overvoltage is supplied to the RFID tag.
If the overvoltage is supplied to the RFID tag, the driving elements in the RFID tag can malfunction or be damaged.
To prevent this, the RFID tag has a limiter to control the voltage input to the driving elements. If the input voltage is lower than a reference voltage to drive the driving elements, the limiter allows the maximum current to flow into the driving elements. If the input voltage is greater than the reference voltage, the limiter allows the minimum current to flow into the driving elements.
The limiter has plural metal oxide semiconductor field effect transistors (MOSFETs), and the MOSFETs are connected to one another in series. If the input voltage is over the reference voltage, the MOSFETs are sequentially turned on, and if the input voltage is below the reference voltage, the MOSFETs are turned off. In other words, if the input voltage is below the reference voltage, the limiter minimizes a leakage current to increase the input current provided to the driving elements. And, if the input voltage is over the reference voltage, the limiter increases the leakage current to reduce the input current. Accordingly, the RFID system can normally operate regardless of the intensity of the input voltage.
However, in general, the threshold voltage of the MOSFET is about 0.8 V, so the limiter can not control a voltage below the threshold voltage. Therefore, when controlling the overvoltage, the limiter has a limit to maximize the leakage current.